1. Field of this Invention
This invention relates to an improved process for the recovery of zinc contained in the ferrites produced by roasting zinc concentrates and/or complex concentrates containing zinc.
2. Prior Art
It is known that zinc values may be obtained from zinc sulphide concentrates by roasting the concentrates to produce a calcine comprising zinc oxide, dissolving the oxide in aqueous sulphuric acid to provide a zinc sulphate solution which is subsequently electrolized to yield the desired zinc value. The oxygen which is free during the electrolysis regenerates sulphuric acid which may be recycled to dissolve additional zinc oxide. This return acid is commmonly called "spent electrolyte" or "return acid".
During the roasting of zinc sulphide concentrates, a portion of zinc, as well as other heavier metal constituents, such as copper, cadmium and lead, combine with a portion of the iron contents thereof to form "ferrite" compounds which, generally speaking, are believed to correspond to the formula MeO.Fe.sub.2 O.sub.3, in which Me may be any of the above-named metals.
Zinc ferrites, ZnO.Fe.sub.2 O.sub.3, are insoluble in the aqueous solution of sulphuric acid used in the leaching of zinc oxide under conditions normally existing in the leaching operations of electrolytic zinc recovery processes. The higher the iron contents in the zinc ore concentrate, the greater the amount of zinc converted into the insoluble form during roasting. Consequently, a residue is obtained from the leaching stage which has a zinc content of about 18 to 20 percent, the larger portion of which is in the form of zinc ferrite, and the remainder comprising unroasted zinc sulphide. Zinc ferrites are accompanied by copper and cadmium ferrites, both of which have not been solubilized during the leaching stages.
Various processes have been suggested for treatment of these residues. All of them can be reduced to two processes: the separation of iron as FeO(OH), or goethite, and the separation of iron as complex basic sulphate (jarosite type) Me.sub.2 Fe.sub.6 (SO.sub.4).sub.4 (OH).sub.12, in which Me can be Na.sup.+, K.sup.+, NH.sub.4 .sup.+, Li.sup.+, or H.sub.3 O.sup.+. Consideration will only be given herein to processes for iron separation as jarosite.
It is known that present-day technology on this process for separating Fe as complex basic sulphate is to some extent conditioned to the composition of the original zinc concentrate. The presence of significant quantities of lead and silver in zinc concentrates determines the practical flow-sheet to be followed in a given plant.
As is already known, the final residue resulting from the decomposition of zinc ferrites is a product which virtually contains all the lead and silver present in the original zinc concentrate. Consequently, it is of interest that in some plants a flow-sheet is followed in order to permit the isolated recovery of this very interesting product, interesting from the economical point of view.
On the other hand, up to now, both in the relevant specifications of patents and in the literature on iron precipitation from solutions resulting from the dissolution of zinc ferrites and other metals present in residues of zinc electrolytic plants as complex basic sulphates of the jarosite type takes place in a slightly acid solution with a pH of about 1.5, as can be gathered, e.g., from Spanish Patent Specification No. 304,601, Norwegian Patent Specification No. 108,047 and Australian Patent Application No. 57013/65.
Under these conditions, it is probable to encounter settling and filtering problems, due to the risk of forming varying amounts of Fe(OH).sub.3 together with the jarosite precipitate, as well as problems encountered due to the silica contents of these solids, when operating at an industrial scale with weak acid solutions containing high iron concentrations,
On the other hand, the amount of calcine used to gradually reduce the acidity of the solution resulting from dissolution of the ferrites down to a pH of about 1.5, represents a very considerable percentage of the total calcine involved, such calcine amount being used in acidity conditions in which the ferrites are not dissolved; consequently, the zinc percentage contained in the residue increases considerably, which results in a simultaneous drop in total zinc recovery.
The problem of zinc contents waste in final plant residue becomes really serious and deserves particular attention in plants where the zinc concentrates processed do not contain high enough lead and silver values as to make it worthwhile to set up a recovery stage for these two metals in the manufacturing process, or even that in spite of lead and silver being present in significant quantities to be of interest, they do not have a recovery stage in order to keep down equipment investment, and, therefore, lead, silver and iron separation is carried out in a single stage (frequency batchwise), finalizing the leaching operation at a pH of 5 to 5.5, as disclosed, e.g., in German Patent Specification No. 1,948,411-8.
Plants operating this way usually eliminate most of the iron as complex basic sulphate, leaving at the end a small quantity of iron which is eliminated as Fe(OH).sub.3 so that impurities, such as As, Sb, Ge, etc., are removed from the solution, which, after being previously settled and freed from solids, is sent to the next purification step.
Zinc contents in residues of plants using this process usually range between 10 and 15 percent, and total recovery, for a 10 percent Fe contents in zinc concentrate, does not usually exceed 90%.
Applicants' novel process, the subject matter of this application, advantageously obviates these problems and leads to very satisfactory results which are, in any case, far better than any results previously achieved.
Our prior U.S. Pat. No. 3,434,798 (based on a Spanish application which matured into Spanish Patent No. 304,601) provides a process for resolving zinc from zinc ferrite. Basically, the process is conducted at atmospheric pressuure and at a temperature within the range 90.degree. to 95.degree. C. which comprises:
1. Treating said residue with a sulfuric acid solution having an initial concentration of not less than 300 g./l. of H.sub.2 SO.sub.4 thereby dissolving said residue to obtain a resulting solution containing zinc sulphate and iron sulphate, said resulting solution having a sulfuric acid concentration of 180 to 200 g./l.,
2. Separating undissolved residue containing said heavier metals from said solution,
3. Adding a diluent to said solution so as to obtain a more dilute solution based on the content of said iron sulphate,
4. Neutralizing said solution to an acidity of about 3 to 5 g./l. of H.sub.2 SO.sub.4, so as to form a granular basic iron sulphate precipitate and a zinc sulphate solution, and
5. Separating said basic iron sulphate precipitate from said zinc sulphate solution.
According to U.S. Pat. No. 3,434,798, the solution resulting from the attack on the residues is neutralized either with sintered blends or zinc calcine to an acidity of 3 to 5 gr/liter of sulfuric acid at a temperature of 90.degree. to 95.degree. C.
With these conditions of acidity and temperature, a high percentage of the copper dissolved during the attack of residues and later neutralization, remains in solution. Normally, this acid solution is sent in all electrolytic zinc plants which have adopted this process, to the neutral leaching stage, where the copper is again precipitated in a variable, but normally high, percentage, with basic salts.
Even when these neutral residues are again attacked by a hot, strongly acidic leaching and the copper is dissolved once more, there is a blackening of the system from copper, which is reflected by a greater loss of this metal accompanying the residue of base iron sulfate.
The use of metal zinc to bind this copper in acid solution is very dangerous, since solutions are involved which at the same time usually contain variable amounts of arsenic and antimony with the consequent danger of imminent release of AsH.sub.3 and SbH.sub.2.
In our first Certificate of Addition (to Spanish Patent No. 304,601) entitled: "Improvements in or relating to the subject matter of U.S. Pat. No. 304,601", dated 9th July 1966, a complete definition of the term "complex concentrate" is given. Also, therein is described the use of sodium, potassium and ammonium ions, these considered both individually and combined, for iron removal from solutions as complex basic sulphate.
In our second Certificate of Addition (to Spanish Patent No. 304,601), dated 14th November 1970, a process is disclosed for dissolving ferrites in two or more countercurrent stages.
Attention is drawn to Spanish Patent No. 304,601, granted Oct. 15, 1964. Attention is also drawn to U.S. Pat. Nos. 1,069,178, 1,973,295 and 3,482,966.